1. Field of the Invention
This invention relates to an electrophotographic photoconductor, and more particularly to an electrophotographic photoconductor comprising a photoconductive layer and a protective layer formed thereon, which comprises a binder resin and finely-divided particles of at least one metal oxide with the surface thereof being coated with a coupling agent selected from the group consisting of a titanate-type coupling agent, a fluorine-containing silane coupling agent, and an acetoalkoxylaluminum diisopropylate, dispersed in the binder resin.
2. Discussion of Background
Hitherto the following electrophotographic photoconductors have been generally known: an electrophotographic photoconductor in which a photoconductive layer comprising Se or a Se alloy as the main component is formed on an electroconductive substrate; an electrophotographic photoconductor which comprises a photoconductive layer prepared by dispersing an inorganic photoconductive material such as zinc oxide or cadmium sulfide in a binder resin; an electrophotographic photoconductor which comprises a photoconductive layer prepared by using organic photoconductive materials such as poly-N-vinyl-carbazole and trinitro fluorenone or an azo pigment; and an electrophotographic photoconductor which comprises a photoconductive layer prepared by using amorphous silicon.
Reliable electrophotographic photoconductors capable of maintaining high image quality for a prolonged period are now strongly demanded.
The photoconductive layer of the conventional photoconductor, when not protected, but exposed, tends to be physically or chemically damaged by corona charging during the charging thereof, or when brought into contact with other members of a copying machine in the course of copying process. Such a damage shortens the expected life span of the photoconductor.
In order to overcome the above shortcoming, photoconductive layers coated with a protective layer have been proposed; for instance, a photoconductive layer coated with an organic film as disclosed in Japanese Patent Publication 38-15446, a photoconductive layer coated with a layer of an inorganic oxide compound as disclosed in Japanese Patent Publication 43-14517, a photoconductive layer coated with an insulating layer through an adhesive layer as disclosed in Japanese Patent Publication 43-27591, and photoconductive layers coated with an a-Si layer, an a-Si:N:H layer and an a-Si:O:H, respectively, by a plasma or light chemical vapor deposition method as disclosed in Japanese Laid-open Patent Applications 57-179859 and 59-58437.
However, when the protective layer has high resistance, for instance, 10.sup.14 .OMEGA..multidot.cm or more, the residual electric potential of the photoconductor increases and the electric charge is built up during repeated use of the photoconductor. Therefore such a protective layer cannot be practically used.
Some protective layers have been proposed so as to eliminate the above drawbacks; for instance, protective layers which also serve as photoconductive layers as disclosed in Japanese Patent Publications 48-38427, 43-16198 and 48-10258, and U.S. Pat. No. 2,901,348; protective layers containing charge transporting materials such as dyes and Lewis acids as disclosed in Japanese Patent Publication 44-834 and Japanese Laid-open Patent Application 53-133444; and a protective layer containing metals or finely-divided particles of metal oxides, which can serve as a resistance-controlling agent, as disclosed in Japanese Laid-open Patent Application 53-3338.
The above protective layers, however, absorb light, so that the quantity of light which reaches the photoconductive layer decreases. As a result, the photosensitivity of the photoconductor is decreased by the protective layers.
The above shortcoming can be overcome by a protective layer, disclosed in Japanese Laid-open Patent Application 57-30846, in which finely-divided particles of a metal oxide having an average diameter of 0.3 .mu.m or less are dispersed as a resistivity-controlling agent so that the protective layer is substantially transparent to visible light. Therefore, a photoconductor comprising such a protective layer can maintain high photosensitivity. Furthermore, since the photoconductive layer is mechanically strengthened by the presence of the finely-divided particles of a metal oxide therein, the durability of the photoconductor is highly enhanced.
However, it has been found that the so-called image flow is caused when copy making is performed by employing the above photoconductor for an extended period of time, or at high humidities or in an atmosphere in which the humidity is drastically changed.
For preventing the image flow, a method is disclosed in Japanese Laid-open Patent Application 62-295066, in which metals or finely divided particles of a metal oxide to be dispersed in a protective layer are subjected to a surface-treatment so as to impart water-repellency.
This method, however, is still insufficient for preventing the image flow, and moreover some adverse side effects such as increasing of the residual electric potential of the photoconductor are caused.